Digital Equipment Corporation

The DECnet Phase IV Specifications

The Digital Network Architecture (DNA) is a set of specifications that defines DECnet. This page describes a subset of those specifications which define DECnet Phase IV. In addition to the specifications and their descriptions, a small amount of background information about DECnet Phase IV in provided to help choose which specification, if any, might be of interest.

Please be aware that all of the DECnet Phase IV specifications are simple text files. They were written in the dark ages of computer history before the widespread introduction of WYSIWYG editors.


DECnet Phase IV was first released by Digital in 1983 for its VMS and RSX-11 systems. Since then it has been implemented on just about every operating system Digital ever shipped including ULTRIX, VAXeln, RSTS/E, TOPS-20, and TOPS-10; the significant exception was RT-11.

Two of major advancements of DECnet Phase IV over DECnet Phase III were support for larger networks (63 areas of 1023) nodes and the use of Ethernet as a datalink. DECnet Phase III limited the entire network to a maximum of 255 nodes. DECnet Phase III also only supported point-to-point and multi-drop links which meant it could not cope the coming introduction of LANs to the marketplace.


DECnet Phase IV still used DDCMP as the basis for its point-to-point links though DDCMP was enhanced to make better use of high-bandwidth and high-latency links such as satellite links.

However Ethernet was the new datalink of choice for DECnet Phase IV systems. And we had the specification to make sure it worked.

Routing, NSP, and Session Control

DECnet Phase IV forced a radical change in the Routing component of DECnet. Larger networks forced introduction of hierarchical routing while LANs introduced the use of multicast messages.

While Routing underwent large scale changes, the Network Service Protocol (NSP) and Session Control only received a minor facelift from their DECnet Phase III definitions and most of those changes were due to the larger addresses in Routing.

Even though Session Control is defined separately from NSP, DECnet Phase IV implementations rarely code Session Control as a distinct component but instead merge it into their implementation of NSP.

Network Management

One of the important aspects of DECnet has the been the existence of network management tools which act the same independent of the DECnet platform. Having a well defined and complete network management specification has allowed DECnet Phase IV to carry on in this regard.

Network management is not just the ability to manage your own DECnet node but also encompasses remote network management, event logging, and loopback at the datalink and application levels. These capabilities, while more common now, were rare in networking protocols when DECnet Phase IV came out (though DECnet Phase III also contained some of these capabilities).

Maintenance Operations (MOP) is far more important to DECnet than the abstract below indicates. MOP is used to downline load software to routers and servers, even servers that don't run DECnet. MOP also allows network managers to connect routers and servers using a feature of MOP called Console Carrier Requester (CCR).

Application Protocols

There have been three basic applications common to data networks: remote file access, remote terminal access, and electronic mail. While remote file and terminal access are defined by the Digital Network Architecture, electronic mail is not.

Not coincidentally DECnet has remained remarkably consistent in its file transfer and remote terminal access protocol but electronic mail has gone in two directions, VMSmail (also called Mail-11) and Message Router / MAILBUS. Which is why no electronic mail protocols are described here.

The seamless integration of remote file access under VMS (and in the past, RSX-11) has long been one the major successes of DECnet. This is due in no small part to the file access protocol used by DECnet which allows a very rich set of file access functions:

DECnet Phase IV introduced a new remote terminal protocol, CTERM, for use on Phase IV systems. However CTERM never really lived up to its expectations but it did manage to get implemented on most of the major DECnet Phase IV platforms.

CTERM is defined by two specifications:


At the same time that DECnet Phase IV was released, Digital introduced a new LAN based protocol for terminal access called LAT (Local Area Terminal). This protocol was revolutionary and Digital was awarded a patent for it. LAT remains to this day a proprietary protocol for which specifications are not available without license.

In Hindsight

Looking back, most of the architects and implementors of DECnet Phase IV would probably agree to a few changes to the architecture for DECnet Phase IV.

The first change to the architecture would be to not have a DECnet Phase IV system change the Ethernet physical address of the machine. A DECnet Phase IV system currently changes its Ethernet physical address to the form of AA-00-04-00-xx-yy where xx-yy is derived from the Phase IV address of the machine. This allowed for simple router-less LAN operation since the LAN address of the remote could be deduced from its Phase IV address. However this caused many problems on systems running multiple protocols, especially ones that do not support the dynamic switching of the Ethernet physical address of the system.

This became quite apparent when the mechanisms of running DECnet Phase IV over Token Ring (IEEE 802.5) network were being developed. Many Token Ring system also want to their Token Ring physical address to something decided by their network administrator. This is in obvious conflict with how DECnet wants to operate. This resulted in a change to DECnet Phase IV such that on Token Ring the existing physical address remains unchanged even when DECnet is running.

The second change would be to have given DECnet Phase IV a larger address space. While in the early 1980s a 16 bit address space seemed like a huge number, today that number seems quite small. Even the TCP/IP protocol family with a 32-bit address is now beginning to strain the limits of its address space after only a decade.

Indeed, a DECnet Phase IV network would fit easily within an IP Class B network. It is interesting to speculate what effect a larger address space would have had on DECnet Phase IV.

The Last Word

All the protocols mentioned above (except for LAT), even though developed by Digital, are not proprietary to Digital. Anyone can implement them without seeking permission from or paying royalties to Digital.

Eventually a number of enhancements were made to DECnet Phase IV. These enchancements become known as DECnet Phase IV+. DECnet Phase IV+ systems were completely interoperable with DECnet Phase IV. In fact most DECnet Phase IV systems are actually DECnet Phase IV+ systems. Most of the enhancements were performance improvements such as the addition of path splitting in Routing and the addition of delayed ACKs and a out of order cache in NSP. There was, however, one significant new functional enhancement: the addition of proxies in Session Control. Note that none of the DECnet Phase IV+ changes are covered in any of the above specifications.

Comments or suggestions for this page are welcome and may be sent to

Last Updated on November 1st, 1994
Matt Thomas,
Network Operating Systems / Network Engineering
Copyright Hewlett-Packard Company 1994. Digital, the Digital logo, DECnet, Alpha AXP, and AXP are trademarks of Hewlett-Packard Company. All other trademarks are the property of their respective owners.